Elementary chemical reaction definition

One conceivable extrapolation of this definition of concertedness is that every elementary chemical reaction is concerted. Every chemical process may be at least conceptually broken up into a sequence of one, two, or more elementary steps or concerted subprocesses. Therefore a reaction may be termed one-step, two-step, or multi-step, and the term "concerted" becomes a redundancy simply equivalent to "one-step . The frequent use of "concerted and "one-step as interchangeable adjectives is one consequence of this extrapolation. 

Microkinetics as defined by Dumesic, is the examination of catalytic reactions in terms of elementary chemical reactions that occur on the catalytic surface and their relation with each other and with the surface during a catalytic cycle. This definition can easily be expanded into covering non-catalytic systems as well. Microkinetics, for the most part, has focused on analysis or understanding of the reaction mechanism. The approach, however, also holds the promise of being used to aid in the synthesis of new materials. Microkinetic modeling is now an important tool for many of the practicing reaction engineers. This approach enables one to formulate and follow the detailed concentration profile for most if not all of the reaction intermediates. 

Some elementary chemical reactions follow a third order rate expression at all normally accessible experimental conditions, and according to the definitions of molecularity must be classified as termolecular. The most common example is the combination of two atoms in the presence of a third species. The rate expression is r = A ( J)[A] [M] for combination of Hke atoms. A, in the presence of the collider or heat bath species M. These reactions do not occur by the simultaneous collision of all three species, which is a very rare event, but by two bimolecular steps that take place within lpsec of one another. An energy transfer mechanism of the reaction may be written as follows ... 

According to the definition given, this is a second-order reaction. Clearly, however, it is not bimolecular, illustrating that there is distinction between the order of a reaction and its molecularity. The former refers to exponents in the rate equation the latter, to the number of solute species in an elementary reaction. The order of a reaction is determined by kinetic experiments, which will be detailed in the chapters that follow. The term molecularity refers to a chemical reaction step, and it does not follow simply and unambiguously from the reaction order. In fact, the methods by which the mechanism (one feature of which is the molecularity of the participating reaction steps) is determined will be presented in Chapter 6 these steps are not always either simple or unambiguous. It is not very useful to try to define a molecularity for reaction (1-13), although the molecularity of the several individual steps of which it is comprised can be defined. 

As explained before, a chemical reaction can seldom be described by a single elementary step, and hence we need to adapt our definition of activation for an overall reaction. Since we are not particularly interested in the effects of thermodynamics we define the apparent activation energy as... 

Chemical reaction — A process that results in the interconversion of chemical species. Chemical reactions maybe elementary reactions or stepwise reactions. (This definition includes experimentally observable interconversions of conformers.) Detectable chemical reactions normally involve molecular entities, as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e., microscopic chemical events ). See also... 

Yes, and many do. While some chemical reactions may only involve a single step, others may involve ten or more elementary steps. Of course, chemists working in different subfields may have different definitions of what constitutes a step of a reaction, depending on what aspects of the reaction they focus on. 

Even complex chemical reaction mechanisms can be separated into several definite elementary reactions, i. e. the direct electronic interaction process between molecules and/or atoms when colliding. To understand the total process B-fot example the oxidation of sulfur dioxide to sulfate - it is often adequate to model and budget calculations in the climate system to describe the overall reaction, sometimes called the gross reaction, independent of whether the process A Bis going via a reaction chain A C D E. .. Z B. The complexity of mechanisms (and thereby the rate law) is significantly increased when parallel reactions occur A X beside A- C,E- X beside E F. Many air chemical processes are complex. If only one reactant (sometime called an educt) is involved in the reaction, we call it a unimolecular reaction, that is the reaction rate is proportional to the concentration of only one substance (first-order reaction). Examples are all radioactive decays, rare thermal decays (almost autocatalytic) such as PAN decomposition and all photolysis reactions, which are very important in air. The most frequent are... 

One of the most important parts of chemical theory is the division of substances into the two classes elementaiy substances and compounds. This division was achieved in 1787 by the French chemist Antoine Laurent Lavoisier (1743-1794), on the basis of the quantitative studies that he had made during the preceding fifteen years of the masses of the substances (reactants and products) involved in chemical reactions. Lavoisier defined a compound as a substance that can be decomposed into two or more other substances, and an elementary substance (or element) as a substance that can not be decomposed. In his Traite Elemen-taire de Clumie [Elementary Treatise on Chemistry], published in 1789. he listed 33 elements, including 10 that had not yet been isolated as elementary substances, but were known as oxides, the compound nature of which was correctly surmised by Lavoisier. Since the discovery of the electron and the atomic nucleus the definitions of elementary substances and compounds have been revised in the ways presented in the following paragraphs. 

When the reaction path is required over the whole distance from reactants to the transition stmcture, elementary problems may occur when using steepest descent. The main reason for this is the fact that the steepest descent path reaches the minimum from the direction given by the eigenvectors of the smallest eigenvalue of the reactant(s) [31]. This is, in general, not identical with the onset of the chemical reaction that we are interested in The question remains Will other definitions of RPs provide better results when using them within the framework of reaction theories mentioned above, and/or is it possible to find new approaches of reaction theories based on new or further developed definitions of RP ... 

A concerted reaction is one in which the conversion of reactants (R) into the products (P) occurs directly by way of a single transition state (T.S.). An exothermic concerted reaction is represented by the potential energy profile of Fig. 3.1(a). When the conversion of reactants into products proceeds by way of more than one transition state, such that one or more intermediates (I) are formed, the processes are accordingly non-concerted. A two-step process involving one (metastable) intermediate is represented by Fig. 3.1(b). However, since each elementary step of any chemical reaction must be concerted, by definition, then case (b) may be divided into the two concerted sequences ... 

This chapter treats the descriptions of the molecular events that lead to the kinetic phenomena that one observes in the laboratory. These events are referred to as the mechanism of the reaction. The chapter begins with definitions of the various terms that are basic to the concept of reaction mechanisms, indicates how elementary events may be combined to yield a description that is consistent with observed macroscopic phenomena, and discusses some of the techniques that may be used to elucidate the mechanism of a reaction. Finally, two basic molecular theories of chemical kinetics are discussed—the kinetic theory of gases and the transition state theory. The determination of a reaction mechanism is a much more complex problem than that of obtaining an accurate rate expression, and the well-educated chemical engineer should have a knowledge of and an appreciation for some of the techniques used in such studies. 

For our purpose elementary steps can be chosen to include any reaction that cannot be further broken down so as to involve reactions in which the specified intermediates are produced or consumed. Ideally, elementary steps should consist of irreducible molecular events, usually with a molecularity no greater than two. Such steps are amenable to treatment by fundamental chemical principles such as collision and transition state theories. Often such a choice is not feasible because of lack of knowledge of the detailed chemistry involved. Each of these elementary reactions, even when carefully chosen, may itself have a definite mechanism, but theory may be unable to elucidate this finer detail [Moore (2)]. 

LEE, YUAN T. (1936-). Awarded the Nobel prize in chemistry in 19X6 jointly with John C. Polanyi and Dudley R. Herschbach for their contributions concerning the dynamics of chemical elementary processes. A former student of Herschbach. Lee relined molecular-beam and laser techniques, comhining them with theory to perform definitive studies of reactions of individual complex molecules. Lee received his Doctorate from the University of California at Berkeley in 1965. 

For chemical kinetics to be operational and thus Eqs. (2.5) and (2.6) to be valid, Eq. (2.4) must be an elementary reaction. To definitively determine this, one must prove experimentally that Eq. (2.4) and the rate law are valid. 

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